Add an appendix "Resources" in tutorial paper

[venpopov]

Since we mention several times "We do not cover X, there are plenty of resources online", it would be helpful to actually add links to currently useful resources (with a note that these might become outdated in the future...; maybe actually a link to a new section on the bmm website Resources which could be updated even after paper is published).

Things like:

• Stan forums
• brms, posterior, bridgesampling, etc websites and tutorials
• The stan user manual

[venpopov]

I'll update this list when I find useful things:

General

• ...

Getting help

• ...

Diagnostics and convergence

• https://mc-stan.org/misc/warnings.html - convergence diagnostics overview

Inference and model comparison

• ...